A CMP (chemical mechanical polishing) apparatus is used in a process of polishing a surface of a wafer in the manufacturing of a semiconductor device. The CMP apparatus is configured to hold and rotate the wafer with a top ring, and press the wafer against a polishing pad on a rotating polishing table to polish the surface of the wafer. During polishing, a polishing liquid (or slurry) is supplied onto the polishing pad, so that the surface of the wafer is planarized by the chemical action of the polishing liquid and the mechanical action of abrasive particles contained in the polishing liquid.
A polishing rate of the wafer depends not only on a polishing load on the wafer pressed against the polishing pad, but also on a surface temperature of the polishing pad. This is because the chemical action of the polishing liquid on the wafer depends on the temperature. Therefore, in the manufacturing of a semiconductor device, it is important to maintain an optimum surface temperature of the polishing pad during polishing of the wafer in order to increase the polishing rate of the wafer, and to keep the increased polishing rate constant.
From this viewpoint, a pad-temperature regulating apparatus is conventionally used to regulate a surface temperature of a polishing pad. FIG. 25 is a schematic view of a conventional pad-temperature regulating apparatus. As shown in FIG. 25, the pad-temperature regulating apparatus includes a pad contact member 111 which is to make contact with a surface of a polishing pad 103, and a fluid supply pipe 112 coupled to the pad contact member 111. The fluid supply pipe 112 branches into a hot water supply pipe 115 coupled to a hot water supply source, and a cold water supply pipe 116 coupled to a cold water supply source. A hot water valve 120 and a cold water valve 121 are attached to the hot water supply pipe 115 and the cold water supply pipe 116, respectively. Either hot water or cold water is selectively supplied to the pad contact member 111 by closing either the hot water valve 120 or the cold water valve 121.
FIG. 26 is a diagram showing operations of the hot water valve 120 and the cold water valve 121, and showing a change in the surface temperature of the polishing pad 103. The hot water valve 120 and the cold water valve 121 are operated based on the surface temperature of the polishing pad 103. In particular, when the surface temperature of the polishing pad 103 exceeds a preset upper limit, the hot water valve 120 is closed and the cold water valve 121 is opened. Similarly, when the surface temperature of the polishing pad 103 falls below a preset lower limit, the cold water valve 121 is closed and the hot water valve 120 is opened.
However, it takes a certain amount of time to cool down the pad contact member 111, because the hot water remains in the pad contact member 111 and in the fluid supply pipe 112 even after a liquid, to be supplied to the pad contact member 111, is switched from the hot water to the cold water. Similarly, it takes a certain amount of time to warm the pad contact member 111 even after the liquid, to be supplied to the pad contact member 111, is switched from the cold water to the hot water. Therefore, a large overshoot and a large undershoot occur at the change in the surface temperature of the polishing pad 103. As a result, the surface temperature of the polishing pad 103 fluctuates greatly.
FIG. 27 is a graph showing change in the surface temperature of the polishing pad 103 as observed when a target temperature of the polishing pad 103 is set at 60° C. As shown in FIG. 27, the surface temperature of the polishing pad 103 changes greatly with a variation of about 20° C. FIG. 28 is a graph showing change in the surface temperature of the polishing pad 103 as observed after making an adjustment to PID control parameters. Also in this case, the surface temperature of the polishing pad 103 changes with a certain degree of variation. FIG. 29 is a graph showing change in the surface temperature of the polishing pad 103 as observed when the target temperature is changed from 60° C. to 50° C. after making an adjustment to PID control parameters. As shown in FIG. 29, the surface temperature of the polishing pad 103 changes greatly again.
The conventional pad-temperature regulating apparatus thus has the problem that the surface temperature of the polishing pad 103 fluctuates greatly during polishing of a wafer and that a desired polishing rate (also referred to as removal rate) cannot be obtained.